The 650 series of light choppers features a control box with separate chopping heads for remote operation. There are three different models within the range, the 651-1 using a standard dual port chopping head and 652-1 and 652-2 employing a single port micro head for use where space is a prime consideration. All the models in the range offer precision frequency control via digital push-buttons or by the application of an AC signal to the sync input on the control unit. When used in conjunction with a SIGNAL RECOVERY lock-in amplifier, computer control of the chopping frequency can be achieved by use of the lock-in amplifierÕs oscillator as a drive signal for the chopper and controlling the oscillator frequency from the computer. In all cases, frequency accuracy and stability are excellent. A LED indicator on the front panel gives constant indication of frequency lock. This indicator can, however, be extinguished for those measurements that need to be executed in total blackout conditions.